1. Field of the Invention
The present invention relates generally to nuclear reactor fuel assemblies, and more particularly, is concerned with a debris filter bottom grid for a nuclear fuel assembly.
2. Description of Related Art
During manufacture, subsequent installation and repair of components of the nuclear reactor coolant circulation system, diligent effort is made to help assure the removal of all debris from the reactor vessel and its associated systems, which circulate coolant throughout the primary reactor coolant loop under various operating conditions. Although elaborate procedures are carried out to help assure debris removal, experience shows that in spite of the safeguards used to effect such removal, some chips and metal particles still remain hidden in the system. Most of the debris consists of metal turnings, which were probably left in the primary system after steam generator repair or replacement.
In particular, fuel assembly damage due to debris trapped at the lower most grid has been noted in several reactors in recent years. Debris enters through the fuel assembly bottom nozzle flow holes from the coolant flow openings in the lower core support plate when the plant is started up. The debris tends to be engaged in the lower most support grid of the fuel assembly within the spaces between the “egg-crate” shaped cell walls of the grid and the lower end portions of the fuel rod tubes. The damage consists of fuel rod tube perforations caused by fretting of the debris in contact with the exterior of the cladding tubes which sealably enclose the fissile material. Debris also becomes entangled in the lower nozzle plate holes and the flowing coolant causes the debris to gyrate, which tends to cut through the cladding of the fuel rods.
Several different approaches have been proposed and tried for carrying out the removal of debris from the nuclear reactors. Many of these approaches are discussed in U.S. Pat. No. 4,096,032 to Mayers et al. Others are illustrated and described in the various patents cross referenced in U.S. Pat. No. 4,900,507, assigned to the instant Assignee. While all of the approaches described in the cited patent and cross references operate reasonably well and generally achieve their objective under the range of operating conditions for which they were designed, a need still exists for a further improved approach to the problem of debris filtering in nuclear reactors, to address an improved reduction in pressure drop across the bottom nozzle that is required for more advanced fuel designs currently going under development and to address a stress corrosion cracking problem that has been experienced in some of the operating debris filtering grids; mainly those known as P-grids. That improvement is addressed in part in co-pending application Ser. No. 10/932,908, filed Sep. 2, 2004, and Ser. No. 10/51,349, filed Jan. 5, 2004. A further need exists to trap even smaller debris without substantially increasing the pressure drop across the fuel assembly while overcoming the fatigue problems experienced by the current P-grids due to flow induced vibrations and stress corrosion cracking due to the high stresses that are induced during manufacture.